1. Field of the Invention
The present invention relates to a scale device such as a position detector, digital scale device, encoder or the like for use on a machine tool, industrial machine, precision machine or the like to detect positional information such as a relative travel, position and the like of a moving part of such machines.
2. Description of the Related Art
Referring now to FIG. 1, a conventional scale device is schematically illustrated in the form of a partially fragmentary perspective view. Generally, the scale device, generally indicated with a reference number 100, is used on machine tools, industrial machines or precision machines of various types to detect positional information such as a relative travel, position or the like of a moving part such as a table, and the position of the moving part is controlled on the basis of a detection output from the scale device 100. As shown in FIG. 1, the scale device 100 includes a scale unit 103 having housed in a case member 102 thereof a long scale member 101 having optical, magnetic or mechanical position signals provided thereon, and a sensor unit 104 provided opposite to the scale member 101.
In the scale device 100, the scale member 101 is formed from a long plate of glass, for example, and has position signals provided longitudinally on one of the main sides thereof, which will not be described in detail herein. On the other hand, the case member 102 is formed from an aluminum alloy to have a generally U-shaped section, and somewhat longer than the scale member 101, for example. The case member 102 has the scale member 101 fixed inside it by bonding. The scale member 101 is fixed to the case member 102 by bonding with an elastically deformable silicone adhesive, for example. Thus the scale member 101 is protected. The scale device 100 has a mounting hole 105 formed in each of longitudinal end portions of the case member 102. With a mounting screw (not shown) driven in each mounting hole 105, the scale unit 103 is fixed to a first part of a machine on which the scale device 100 is to be used, for example, a stationary part.
In the above scale device 100, the sensor unit 104 is built in the scale member 101. The sensor unit 104 has a bearing mechanism provided on a slider. The slider has a substrate having a sensor mounted thereon, which is not illustrated and described herein. The bearing mechanism enables the slider to be freely slid longitudinally by the scale member 101 as a guide. The sensor unit 104 has provided thereon a coupling unit which is not described in detail herein. The coupling unit is held within a longitudinal guide opening 106 formed in the case member 102 opposing the scale member 101. The coupling unit extends out for integration with a mounting unit 107 provided thereon to form a detector unit 108.
The mounting unit 107 of the scale device 100 has a mounting hole 109 formed therein at either longitudinally opposite end thereof. With a mounting screw (not shown) driven in the mounting hole 109, the mounting unit 107 is fixed to a second part of the machine, for example, a moving part such as a moving table. The sensor unit 104 is electrically connected to an internal circuit unit such as a control circuit etc. incorporated in the mounting unit 107 via a flexible cable. The internal circuit unit in the mounting unit 107 is further connected to a controller, display unit, etc. of the machine via a cable 110 and connector 111 to provide information about the position signal detected by the sensor as an output. It should be noted that in the scale device 100, there may be provided on the mounting unit 107 a connector via which the internal circuit unit is connected to an external controller, display unit, etc. of the machine, for example.
Being interlocked with the moving part of the machine, the sensor unit 104 is moved by the mounting unit 107 in relation to the scale unit 103. The sensor in the sensor unit 104 detects the position signals provided on the scale member 101 of the scale unit 103 and supplies the detection output to the machine via the mounting unit 107 for controlling the moving part and for display on the display unit. It should be noted that in the scale device 100, the guide opening 106 formed in the case member 102 is closed along the whole length thereof by seal lip members (not shown) formed from rubber or the like and which are put in elastic contact with the outer surfaces of the coupling unit to prevent dust or the like from adhering to the scale member 101.
In the scale device 100, the scale member 101 required to be of a high precision is formed from glass and the case member 102 required to have excellent mechanical rigidity and workability is formed from an aluminum alloy. Namely, the scale member 101 is formed from one material while the case member 102 is formed from another material. They are fixed to each other by bonding with an adhesive which is elastically deformable. In addition, only the case member 102 is fixed with mounting screws to the mounting portion of a machine. Generally, the mounting portion of the machine is formed from a steel.
On the other hand, the scale member 103 of the scale device 100 is installed to the first part of the machine while the mounting unit 107 of the detector unit 108 is installed to the second part, moving in relation to the first part, of the machine as having previously been described. That is, the members, formed from different materials, of the scale device 100 are installed to the corresponding mounting portions, also formed from different materials, of the machine.
Generally, the glass used to form the scale member 101 has a linear expansion coefficient of about 8 to 9×10−6K−1, while the aluminum alloy as the material of the case member 102 has a linear expansion coefficient of about 23 to 24×10−6K−1. That is, the scale member 101 and case member 102 become largely different in dimensions from each other due to expansion or shrinkage depending upon the environmental conditions such as temperature, humidity, etc. In this scale device 100, a layer of the elastically deformable adhesive is used to absorb the dimensional difference between the scale and case members 101 and 102.
On the other hand, the steel used to form the machine has a linear expansion coefficient of about 11 to 12×10−6K−1. Since the case member 102 of the scale device 100 is fixed with the mounting screws to the mounting portion of the machine, the dimensional difference between the case member 102 and mounting portion of the machine due to expansion or shrinkage depending upon the environmental conditions such as temperature, humidity, etc. will be limited. Therefore, the scale member 101 of the scale device 100 will uniquely be expanded or shrunk depending upon the environmental conditions, so that the scale member 101 can hardly be positioned with a high precision in relation to the mounting portion of the machine. Thus, it is necessary to make a readjustment such as positioning the scale member 101 in relation to the sensor unit 104 with the scale unit 103 and detector unit 108 left installed on the machine. The readjustment is very troublesome. Also, in the scale device 100, the accuracy of measurement is lower depending upon environmental conditions, which will cause the machining precision of the machine to be lower.